The present invention relates generally to systems and methods for manufacturing absorbent garment cores. More specifically, the present invention relates to a system and method for providing precise disposition of superabsorbent particles and other particulate and fibrous additives into an absorbent core.
Disposable absorbent garments such as infant diapers or training pants, adult incontinence products and other such products typically were constructed with a moisture-impervious outer backsheet, a moisture-pervious body-contacting inner topsheet, and a moisture-absorbent core sandwiched between the liner and backsheet.
Much effort has been expended to find cost-effective materials for absorbent cores that display good liquid absorbency and retention. Particles of superabsorbent materials (SAP) in the form of granules, beads, fibers, bits of film, globules, etc., have been favored for such purposes. Such SAP materials generally are polymeric gelling materials that are capable of absorbing and retaining even under moderate pressure large quantities of liquid, such as water and body wastes, relative to their weight. The SAP particles typically have been distributed within a fibrous web of fluffed pulp material, which may comprise natural or synthetic fibers. Such absorbent structures are commonly referred to as fluff pulp/SAP cores.
Superabsorbent material generally is a water-insoluble but water-swellable polymeric substance capable of absorbing water in an amount that is at least ten times the weight of the substance in its dry form. In one type of superabsorbent material, the particles may be described chemically as having a back bone of natural or synthetic polymers with hydrophilic groups or polymers containing hydrophilic groups being chemically bonded to the back bone or an intimate admixture therewith. Included in this class of materials are modified polymers such as sodium neutralized cross-linked polyacrylates and polysaccharides including, for example, cellulose and starch and regenerated cellulose that are modified to be carboxylated, phosphonoalkylated, sulphoxylated or phosphorylated, causing the SAP to be highly hydrophilic. Such modified polymers also may be cross-linked to reduce their water-solubility.
The ability of a superabsorbent material to absorb liquid is dependent upon the form, position and/or manner in which particles of the superabsorbent material are incorporated into the fibrous web of the absorbent core. Whenever a particle of the superabsorbent material is wetted, it swells and forms a gel. Gel formation can block liquid transmission into the interior of the absorbent core, a phenomenon called xe2x80x9cgel blocking.xe2x80x9d Gel blocking prevents liquid from rapidly diffusing or wicking past the xe2x80x9cblockingxe2x80x9d particles of superabsorbent, causing portions of a partially hydrated core to become inaccessible to multiple doses of urine. Further absorption of liquid by the absorbent core must then take place via a diffusion process. This is typically much slower than the rate at which liquid is applied to the core. Gel blocking often leads to leakage from the absorbent article well before all of the absorbent material in the core is fully saturated.
Despite the incidence of gel blocking, superabsorbent materials are commonly incorporated into absorbent cores because they absorb and retain large quantities of liquid, even under load. However, in order for superabsorbent materials to function, the liquid being absorbed in the absorbent structure must be transported to unsaturated superabsorbent material. In other words, the superabsorbent material must be placed in a position to be contacted by liquid. Furthermore, as the superabsorbent material absorbs the liquid it must be allowed to swell. If the superabsorbent material is prevented from swelling, such as by being tightly constrained within the fibrous web or by pressure exerted by the swelling of adjacent superabsorbent particles, it will cease absorbing liquids.
Adequate absorbency of liquid by the absorbent core at the point of initial liquid contact and rapid distribution of liquid away from this point are necessary to ensure that the absorbent core has sufficient capacity to absorb subsequently deposited liquids. Previous absorbent cores have thus attempted to absorb quickly and distribute large quantities of liquids throughout the absorbent core while minimizing gel blocking during absorption of multiple doses of liquid.
Some of the more important performance attributes of an absorbent core of a diaper (or any other absorbent garment) are functional capacity, rate of absorption, and core stability in use. Absorption under load or AUL is a good measure of functional capacity and the rate at which that absorption occurs. AUL is a function of both SAP basis weight (mass per unit area) and the composition of SAP used in the composite. Conventional baby diaper cores that contain only a fibrous web of fluff pulp and a high gel strength SAP typically maintain adequate SAP efficiency if the core contains less than about 50% SAP. Fluff/SAP diaper cores containing more than 50% SAP generally result in lower SAP efficiency because of gel blocking. Although fluff/SAP cores at greater than 50% SAP can provide adequate absorbency, the overall basis weight of the core typically must be increased to compensate for the lower efficiency of the SAP. Increasing the basis weight decreases the performance/cost ratio of the absorbent core, making them uneconomical. Also, increased basis weights tend to affect the fit and comfort of the garment, as well as impacting the packaging and shipping costs.
Attempts to increase the relative weight of SAP by reducing the basis weight of the conventional fluff pulp have resulted in failure because low density fluff pulp mats have been unable to withstand the tensile loads placed on them during the manufacturing process. Such cores also exhibit poor wet strength, making them unstable during use, and fail to adequately secure the SAP in place. The introduction of relatively high integrity fibrous structure cores, however, has allowed the basis weight of the fibrous web to be decreased without compromising the manufacturability and wet strength of the absorbent core. These absorbent core structures have improved SAP efficiency and a lower overall basis weight. Such absorbent cores are disclosed, for example, in U.S. Statutory Invention Registration No. H1,565 to Brodof et al., which is incorporated by reference herein in its entirety and in a manner consistent with the present invention. These high integrity fibrous structure cores, referred to herein as xe2x80x9ctow/SAPxe2x80x9d cores or xe2x80x9ctow-basedxe2x80x9d cores, typically use a continuous tow of crimped filaments. The tow may be provided to the absorbent core manufacturer in a compact form and xe2x80x9copenedxe2x80x9d (i.e., xe2x80x9cbloomedxe2x80x9d or fluffed up) prior to being assembled into an absorbent core.
In some cases, the fibrous web of the tow/SAP core may be treated with a tackifying agent to adhere the SAP particles to the fibrous web. In other cases, the SAP particles may be introduced into the fibrous web without any adhesive, binder or tackifying agent, such as is disclosed in U.S. Pat. No. 6,068,620 issued to Chmielewski et al., which is incorporated by reference herein in its entirety and in a manner consistent with the present invention. Such a construction has been referred to as a dry-formed composite (DFC) core. A DFC core may be surrounded by a tissue layer or multiple tissue layers to form a DFC laminate structure that contains the fibrous web and SAP.
A problem with SAP-containing fibrous cores has been to provide the SAP into the fibrous web in a controlled manner. Typical known processes for creating a conventional fluff pulp/SAP core use a large forming chamber to blend the SAP with the fluffed pulp, then convey this blend onto a drum or screen by using a vacuum. The drum or screen has forming pockets that form the fluff pulp/SAP material into the desired shape and the formed cores then are deposited for integration into absorbent products. Such methods have been found to be inefficient during startup and transitions in the manufacturing line speed because they require a relatively large amount of time to provide a stabilized mixture of SAP and fluff pulp, leading to the creation of a large number of scrap products until stabilization.
Other conventional processes for forming fluff pulp/SAP cores immerse the fluffed pulp in a fluid mixture containing SAP particles, then dry the fluff pulp/SAP mixture before integration into the absorbent article. Such wet forming processes typically require more manufacturing steps and are more expensive than dry forming methods.
Other feeding systems use fixed-size moving mechanical gates that provide a uniform amount of SAP to the absorbent core, such as is disclosed in U.S. Pat. No. 6,139,912 to Onuschak et al., which is incorporated herein by reference in its entirety and in a manner consistent with the present invention. Although such devices may be suitable for providing an even flow of SAP or other powdered and particulate additives to absorbent cores, they rely on relatively complex feeding machinery, including a rotary valve that uses a pneumatic SAP conveyor to return undistributed SAP back to a supply container. Pneumatic conveyors typically require a relatively long time to become pressurized and to convey the SAP, causing inefficiencies during transitional phases, such as when the machine operating speed varies, such as during start-up and shut-down, or when it is desired to change the amount of SAP being fed to the core. The additional parts of such feeders may also be expensive and subject to wear and other service problems. Similar devices, having similar deficiencies, are disclosed in U.S. Pat. No. 4,800,102 to Takada, which is incorporated herein by reference in its entirety and in a manner consistent with the present invention.
Still other feeding systems use pneumatic particle projectors that use pressurized gas to convey the SAP to the surface of the absorbent core. Such devices are disclosed, for example, in U.S. Pat. No. 5,614,147 to Pelley and U.S. Pat. No. 5,558,713 to Siegfried et al., which are incorporated herein by reference in their entirety and in a manner consistent with the present invention. Such systems rely on relatively complex air conveyors, that may be susceptible to blockage and may not efficiently accommodate as wide a variety of particulate, powder and fibrous materials as other systems due to their relatively small passage sizes. Indeed, it has been found that the compressed air used in such pneumatic conveyors is often contaminated with oil that may cause blockage, SAP degradation, and other problems. Such systems may also require a relatively long time to stabilize, leading to inefficiencies during transitional phases.
Other known SAP feeding systems are disadvantageous for a number of reasons. First, the mixture of fiber and SAP still is subject to local concentrations and shortages of SAP. Second, these feeding systems typically can not be controlled accurately enough to provide concentrations and shortages of SAP when they are desired. Third, such feeding systems can not be controlled to accurately provide reduced SAP amounts that are necessary during transitional phases, leading to improperly loaded cores during those phases of operation.
These are just a few of the disadvantages of the prior art which the preferred embodiments seek to address. The foregoing description of certain material, methods and systems with their attendant disadvantages in no way is meant to infer that the present invention excludes such materials, methods, and systems. Indeed, certain embodiments of the invention solve some of the aforementioned disadvantages, yet utilize the same or similar materials, methods and/or systems.
It would be desirable to provide an apparatus and method for dry forming absorbent cores. It also would be desirable for such an apparatus and method capable of providing a homogeneous mixture of particulate matter and fibrous material. It also would be desirable for such an apparatus and method to allow relatively precise positioning of regions of high particulate matter concentrations within the fibrous material to provide zoned properties to the absorbent cores. Still further, it would be desirable for such an apparatus and method to be efficient, easy to operate, and capable of operating at high line speeds.
In accordance with these and other features of various embodiments of the invention, there is provided an apparatus and method for dry forming absorbent cores. In accordance with one embodiment of the invention, the apparatus has a rotatable drum having a substantially cylindrical surface. A vacuum surface having one or more holes is located substantially circumferentially around the substantially cylindrical surface. A vacuum chamber is located within the rotatable drum, and has one or more vacuum passages forming a vacuum zone that lies subadjacent at least a portion of the vacuum surface. A first casing sheet supply overlies the vacuum surface at a first location, and a supply of fibrous material overlies the first casing sheet supply at a second location. A supply of particulate matter is deposited onto the supply of fibrous material at a third location, and a second casing sheet supply overlies the first casing sheet supply, supply of fibrous material and supply of particulate matter at a fourth location, thereby forming an absorbent core composite.
In accordance with other features of preferred embodiments of the invention, the particulate matter is a superabsorbent polymer, and the fibrous material is tow fibers, preferably a cellulose acetate tow. In accordance with another preferred embodiment of the invention, the supply of opened tow is provided from a tow forming jet, and the supply of opened tow exits the forming jet at a break angle. In accordance with another preferred embodiment, the particulate matter is provided from a vibratory feeder. In still another preferred embodiment, the mixture of superabsorbent particles and opened tow is at least about 30% by weight superabsorbent particles.
In accordance with yet another embodiment of the invention, there is provided a method for dry forming absorbent cores. The method includes rotating a drum having a substantially cylindrical surface and a vacuum surface, the vacuum surface comprising one or more holes and being disposed substantially circumferentially around the substantially cylindrical surface. A vacuum is applied to a vacuum chamber located within the drum. The vacuum chamber has one or more vacuum passages forming a vacuum zone subadjacent at least a portion of the vacuum surface. A first casing sheet supply is applied to overlie the vacuum surface at a first location, and a supply of fibrous material is applied to overlie the first casing sheet supply at a second location. A supply of particulate matter is deposited onto the supply of fibrous material at a third location, and a second casing sheet supply is applied to overlie the first casing sheet supply, supply of fibrous material and supply of particulate matter at a fourth location, thereby forming an absorbent core composite.
In accordance with other features of preferred embodiments of the inventive method, the particulate matter is a superabsorbent polymer, and the fibrous material is opened tow, preferably an opened cellulose acetate tow. In accordance with still other preferred embodiment of the inventive method, the supply of opened tow is provided from a tow forming jet and the particulate matter is provided from a vibratory feeder.
These and other features of the invention will be readily apparent from the Detailed Description that follows, along with reference to the drawings appended hereto.